The present invention relates to a method for model-based determination of the fresh air mass flowing into the cylinder combustion chamber of an internal combustion engine during an intake phase. The total fresh air quantity present in the combustion chamber after termination of the intake phase may subsequently be calculated on the basis of the ascertained fresh air mass flowing into the cylinder combustion chamber.
When controlling internal combustion engines, particularly when controlling gasoline engines for motor vehicles, precise knowledge of the air mass flowing into the cylinder combustion chamber(s), and/or of the air mass present in the combustion chamber after termination of the intake phase, plays a decisive role. This is because important variables such as injection duration and ignition angle are calculated on the basis of the air mass (load) present in the combustion chamber after termination of the intake phase. Errors in the load signal rapidly result in emission, consumption, and drivability problems. Because in practice there are typically no sensors for direct measurement of the air mass flow at the intake manifold outlet, at the transition to the cylinder inlet, load models in which the load is calculated on the basis of other existing measured variables, are typically used. Such load models must be very precise, in particular if there is no air mass sensor (e.g., HFM sensor) in the entire intake system, on the basis of whose measured values possible model errors may be regulated out using a regulator. In load modeling, the air mass flowing into the cylinder is usually used as a function of the pressure in the intake manifold and further parameters such as speed of the internal combustion engine, position of the camshaft, temperatures inside the intake-combustion-exhaust path, and/or the exhaust gas counter pressure. A linear model, in which the load rises linearly with the intake manifold pressure (linear model) is frequently used for the description of the dependence of the load on the existing intake manifold pressure. The position of the straight lines is determined by the remaining input variables described above.
In model-based load acquisition, the load signal is typically calculated online in the engine controller using the existing load model from a value of the intake manifold pressure. The required intake manifold pressure may be measured or may also be ascertained based on a model.
In addition to the load acquisition, the load model is also used for the load controller. In the load controller, the setpoint intake manifold pressure is calculated from the load setpoint value, which is predefined from the torque structure (by the driver command). In the case of a linear load model, this is possible without problems because of the ability to simply invert linear functions. The setpoint intake manifold pressure is used (if necessary together with the load setpoint value and a possibly existing tank ventilation mass flow) for calculating the throttle valve setpoint position (throttled operation) and/or for calculating the gas exchange valve control times and/or stroke paths (unthrottled operation). If setpoint value and actual value of the intake manifold pressure correspond, because of the ability to invert the linear load model, load actual value and load setpoint value (filling actual value and filling setpoint value) are equal. This is a necessary condition for the functioning of the torque structure.
From the findings described, it results as a requirement for the use of a linear load model for determining and controlling the load, that the cylinder filling actually rise linearly with the intake manifold pressure. Particularly in turbocharged, direct-injection internal combustion engines, a linear relationship between intake manifold pressure and load does not occur over the entire pressure range. In these engines, the load rises disproportionally with increasing “unthrottling” (via throttle valve and/or via gas exchange valve strokes or gas exchange valve control times). Furthermore, in turbocharged, direct-injection internal combustion engines, overflow of the fresh air intake is intentionally caused at some operating points, such that fresh air is directed uncombusted from the intake manifold into the exhaust gas system during the intake phase. The goal of this being to increase the air-mass flow and achieve a more favorable operating point of the turbocharger. In the case when overflow is used, the fresh air mass intake is therefore unequal to the fresh air mass remaining in the cylinder combustion chamber after termination of the intake phase.
Therefore, a differentiation is made in the following in the meaning of the present invention, and within the scope of this disclosure “load” refers to the fresh air mass located in the cylinder after termination of the intake phase, and “fresh air mass intake” refers to the entire air mass taken in during the intake phase.
Both the nonlinear increase of the load and also the overflow of the fresh air mass intake may not be taken into consideration adequately using a linear load model. Especially in turbocharged, direct-injection internal combustion engines, large modeling errors, which must be regulated out permanently on the basis of an air mass meter and a regulator, result if a linear model is used for ascertaining the fresh air intake or for ascertaining the load. Operation without an air mass meter is therefore not possible using a linear model. The use of air mass measuring units in turbocharged internal combustion engines is problematic, however, since situating the measuring units is extremely difficult. If the measuring unit (e.g., HFM sensor) is installed in the high-pressure part of the intake manifold (i.e., after the compressor), the dynamic precision of the measurement is good, but the component stress (e.g., by oil film contamination) is very high and the component service life is too short. If an HFM sensor is situated in the low-pressure part of the intake manifold (i.e., before the compressor) the high component strain may be largely avoided, but the dynamic signal quality is very poor because of the interposed compressor.
The present invention is based on specifying a method for model-based determination of the fresh air mass flowing into the cylinder combustion chamber of an internal combustion engine during an intake phase, which ensures sufficiently precise determination of the inflowing fresh air mass and also ensures the determination of the load even in turbocharged internal combustion engines, without the use of air mass measuring units.
The object is achieved according to the present invention by a method for model-based determination of a total fresh air mass flowing into a cylinder combustion chamber of an internal combustion engine during an intake phase, which includes ascertaining a first partial air mass on the basis of a first load partial model, the first partial load model having linear characteristics, ascertaining a second partial air mass on the basis of a second load partial model, the second load partial model having nonlinear characteristics, and determining the total fresh air mass by adding the previously ascertained partial air masses. By ascertaining the fresh air mass flowing into the cylinder combustion chamber during the intake phase using different load partial models, including a first fresh air component being ascertained via a first load partial model having purely linear functionality and a second fresh air component being ascertained via a second load partial model having nonlinear functionality, more precise acquisition of the inflowing fresh air mass and thus more precise load acquisition and load control can be obtained, in particular in turbocharged internal combustion engines. In the present invention, when reference is made to nonlinear or linear functionality or nonlinear and linear relationship, it is meant that the relationship between the fresh air mass flowing into the cylinder(s) and the intake manifold pressure existing in the intake manifold is ascertained.
The method is preferably used in turbocharged, direct-injection gasoline or diesel internal combustion engines. The inflowing fresh air mass is advantageously determined exclusively via the linear load model up to a first intake manifold pressure defined by a first intake manifold pressure threshold, particularly where Ps1≈0.52×Pa; where Pa=exhaust gas counter pressure. The inflowing fresh air mass is first determined via a combination of the two load partial models (linear base model+nonlinear offset model) upon reaching this pressure threshold.
When determining the inflowing air mass via a combination of the two load partial models, the linear component is ascertained via the first load partial model and the nonlinear component is ascertained separately via the second load partial model, and is added onto the linear component in the form of an offset, according to a preferred embodiment of the invention.
In an especially beneficial refinement of the present invention, a third load partial model is provided, which also has nonlinear behavior, through which a third partial air mass value is ascertained. An embodiment of this type is of interest in particular in cases in which overflow of the fresh air intake occurs during the intake phase. This is the case in turbocharged internal combustion engines at certain operating points, for example. In particular, the overflow fresh air mass component is ascertained via the third load partial model, which may then be determined from a fresh air mass intake via the inlet valve, and thus allows the total fresh air mass (load) intake remaining in the cylinder to be calculated.
The method according to the present invention is preferably used in direct-injection internal combustion engines having a turbocharger.
In the following, the present invention is explained in greater detail with reference to the figures. Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.